Nasopharyngeal temperature probes should be inserted between 9 and 11 cm from the nares to maximize the likelihood of positioning in the upper third of the nasopharynx, which is near the internal carotid artery. (Image source: Thinkstock)

Nasopharyngeal temperature probes should be inserted between 9 and 11 cm from the nares to maximize the likelihood of positioning in the upper third of the nasopharynx, which is near the internal carotid artery. (Image source: Thinkstock)

Temperature monitoring is an ASA standard. Nasopharyngeal temperature probes are commonly used when a patient’s trachea is intubated. How far should the probe be placed? To monitor brain temperature, the probe should be positioned near the internal carotid artery. Dr. Seonghoon Ko, Department of Anesthesiology and Pain Medicine, Chonbuk National University Medical School, Jeonju, Chonbuk, Republic of Korea, and colleagues sought to determine where the probe should be placed to be closest to the internal carotid artery in the nasopharynx using spiral cranial CT images. The results of this clinically useful study are summarized in the article titled “Optimal Nasopharyngeal Temperature Probe Placement,” which was published in this month’s issue of Anesthesia & Analgesia.

By analyzing spiral cranial CT images from 50 men and 50 women, the study authors identified the region in closest proximity to the internal carotid artery to be the upper one-third of the posterior nasopharyngeal mucosa. They followed this preliminary investigation with a prospective study of 370 elective surgical patients who received general anesthesia and nasopharyngeal temperature probe insertion. Thirty minutes to one hour after the induction of anesthesia, the data-collecting anesthesiologist evaluated the placement of each temperature probe using nasendoscopy. The probes were then moved to suboptimal locations and the difference in temperature between the initial and final probe location was recorded.

The results indicate that the probe should be inserted between 9 and 11 cm from the nares to maximize the likelihood of positioning in the upper third of the nasopharynx. The good news is that slight displacements from optimal position have minimal impact on the accuracy of temperature measurement. The median temperature differences between the upper nasopharynx and the non optimal sites were between 0.0° C and 0.2° C. However, the maximum recorded temperature differences were 0.7° C in the nasal cavity, 0.2° C in the lower nasopharynx, 0.3° C in the oropharynx, and 0.8° C in the hypopharynx. To ensure the probe passes beyond the nasal cavity, it should be placed at a depth of at least 8 cm, and to ensure it does not reach the hypopharynx, the insertion depth should be limited to 14 cm or less.  Measurement and marking may be necessary prior to insertion.